1. Field of the Invention
This invention relates to a method for preparing silicone foams.
2. Description of the Prior Art
Organopolysiloxane compositions that cure while simultaneously foaming (hereinafter abbreviated as foaming/curing organopolysiloxane compositions) are well known and are used, for example as sealants for interpenetrating elements in nuclear power plants. These types of organopolysiloxane compositions are typically divided into two different fluids, a base and a curing agent, for storage. When the two parts are mixed, curing occurs accompanied by foaming based on the evolution of gaseous hydrogen to produce silicone foam.
Silicone foam gaskets that are formed at the particular sealing site from a foaming/curing organopolysiloxane composition have recently entered into use (Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3-94876 (94,876/1991)). These silicone foam gaskets are used in various applications, such as automotive headlamps and waterproof casings for holding automotive electronic components and the like. One problem with such Organopolysiloxane compositions is that they may not produce silicone foam gaskets having uniform cells when the two fluid parts are mixed in a closed or sealed system. A particular problem that occurs when the organopolysiloxane composition has a high viscosity is that the silicone foam obtained by foaming and curing the composition has irregular cells.
When this type of organopolysiloxane composition is used for the fabrication of foam gaskets, the composition is mixed in a mixer and extruded in bead form. Since the bead shape must be maintained after until foaming has begun, the composition must have a high viscosity, with the result, as discussed above, that the silicone foam gasket having uniform cells cannot be obtained.
Methods that solve the above-described problems have already been provided by the inventors in Japanese Patent Application Laid Open (Kokai or Unexamined) Numbers Hei 5-31814 (31,814/1993) and Hei 6-200066 (200,066/1994). These methods provide for the fabrication of silicone foam having uniform cells by quantitatively feeding the base, curing agent, and air into a special type of mixing device and mixing therein. These methods, however, are encumbered by several disadvantages from a production standpoint. Thus, they require the quantitative dosing of highly compressible air into the mixing device, which necessitates a special control device of complex structure. In addition, a very delicate control of the gas feed must be exercised in order to obtain silicone foam having uniform cells.
The inventors achieved the present invention as the result of extensive investigations directed to solving the problems described above.
In specific terms, the object of the present invention is to provide a highly productive method for the preparation of silicone foam having uniform cells.